cmd/compile: break apart dumptypestructs
    
    dumptypestructs did several different jobs.
    Split them into separate functions
    and call them in turn.
    
    Hand dumptypestructs a list of dcls,
    rather than reading the global.
    
    Rename dumpptabs for (marginal) clarity.
    
    This is groundwork for compiling autogenerated
    functions concurrently.
    
    Passes toolstash-check.
    
    Change-Id: I627a1dffc70a7e4b7b4436ab19af1406267f01dc
    Reviewed-on: https://go-review.googlesource.com/41501
    Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>